2 Meiosisthe process of nuclear division within a cell that involves in a reduction of chromosome number from diploid to haploid, resulting in gamete formationHuman somatic (body) cells have 46 chromosomes22 pairs of autosomes and 2 sex chromosomesXX in females, XY in malesHuman gametes (sex cells – eggs and sperm) have 23 chromosomesSperm cells carry either an X or a Y

3 MeiosisOf the 46 chromosomes, there are 23 kinds - 2 copies of each kindOne copy from mother, one from father, coming together when a sperm fertilizes an eggThe 2 copies are called homologous chromosomes – they carry the same genes (although they might vary in the version – different alleles)Exception: X and Y, although paired in males, are not considered homologous

4 Meiosis First: review of mitosis Prior to mitosis Replication of DNA46 chromosomes all copiedEach copy = chromatid‘sister’ chromatids held together at centromere

8 Stages of MeiosisPrior to meiosis: DNA replication, resulting in 46 chromosomes each with 2 sister chromatids held together at centromeresMeiosis occurs in 2 phases:Meiosis I: Reduction divisionSeparation of homologous chromosomesChromosome number now 23Each chromosome still has 2 sister chromatidsMeiosis II: Mitotic divisionSeparation of sister chromatids

9 Meiosis I In Prophase I, the homologous chromosomes pair upThis ensures that each daughter cell winds up with exactly one copy of each of the 23 kindsThe sister chromatids of the 2 chromosomes make 4 total copies of each DNA strand – a tetradWhile paired up, they entwine (synapsis) and can break and reform, swapping parts (chiasmata and crossing over)

13 Products of Meiosis Four daughter cells Haploid (23 chromosomes)Each has one copy of each kind of chromosomeIdentical, except for 2 things:In males, half X, half Y sex chromosomesCrossing over results in different combinations of alleles

28 Uterine cycle Corresponds with Ovarian Cycle Menstrual phaseSynchronized by ovarian hormonesMenstrual phaseProliferativie phaseCorresponds to ‘diestrous’ in other mammalsSecretory phaseCorresponds to ‘estrous’ in other mammals